Abstract The encapsulated fungus Cryptococcus neoformans (Cn) is the most common cause of fungal meningitis, with the highest rate of cryptococcosis found in AIDS patients. Despite antifungal treatment, cryptococcal meningitis is known for its high morbidity and mortality rates due to its difficult eradication from the brain. Cn?s capsule contributes directly to its pathogenesis; specifically, its main component, glucuronoxylomannan (GXM), has been associated with invasion of the brain. Studies in mouse models of cryptococcal meningoencephalitis have indicated that the portals of Cn invasion into the brain are the cerebral capillaries, which have been confirmed by pathological studies in human brain tissues. Cn can cross the blood brain barrier (BBB) as free yeast either transcellularly using the Trojan horse mechanism, and/or by paracellular passage. However, the fungal factor(s) supporting these invasive processes is/are not well-described. Therefore, the goal of this application is to dissect the cellular and signaling mechanisms by which Cn GXM dysregulates the BBB, allowing the fungus to invade the central nervous system (CNS). Furthermore, microglial (MG) cells are the main form of active immune defense in the CNS. These cells are critical for combating CNS-invading microbes, including Cn, but the specific function(s) of MG cells required to counteract GXM?s BBB damage and Cn invasion is/are poorly understood. Our central hypothesis is that GXM disrupts endothelial cells junctions and inhibits MG cell migration resulting in Cn invasion and colonization of the CNS. To address our hypothesis, we propose the following aims: (1) Determine the role of Cn GXM on BBB physical integrity in vivo; (2) Establish the cell signaling mechanisms by which Cn GXM induces human brain microvascular endothelial cell barrier permeability; and (3) Study GXM- induced inhibition of MG cell migration and control of Cn brain invasion in vivo. Identification of the mechanisms by which Cn GXM alters the BBB integrity and MG cell functions will provide novel insights into the neurotropism of this deadly infection and may offer new therapeutic opportunities and preventive measures for combating cerebral cryptococcosis, a disease that kills ~200,000 AIDS patients annually around the world.